Examination of the Last Large Solar Energetic
Particle Events of Solar Cycle 23
C. M. S Cohen', G. M. M a s o n ^ R. A. Mewaldt', A. C. Cummings',
A. W. Labrador", R. A. Leske", E. C. Stone", M. E. Wiedenbeck", and
T. T. von Rosenvinge
"California Institute of Technology, Mail Code 220-47, Pasadena, CA 91125
Johns Hopkins Applied Physics Laboratory, MS MP3-E128, Laurel, MD 20723
'Jet Propulsion Laboratory, MS 169-327, Pasadena, CA 91109
''NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771
Abstract. The last two large solar energetic particle (SEP) events of solar cycle 23 were
observed in December 2006 by several spacecraft including ACE and STEREO. Active region
number 10930 rotated over the eastern limb of the Sun already generating intense x-ray flares.
As it crossed the disk, it produced 4 X-class flares and at least 3 halo coronal mass ejections.
The two dominant SEP events occurred when the region was at ~E70 and ~W25. We have
combined particle observations from the Solar Isotope Spectrometer (SIS) and the Ultra-Low
Energy Isotope Spectrometer (ULEIS) on ACE and the Low Energy Telescope (LET) on
STEREO for each event. Energy spectra for many heavy ion species integrated over the
duration of each SEP event show distinct differences between the two events. We find the
second event (on December 13) has a much harder spectrum above 10 MeV/nucleon and a 12-60
MeV/nucleon composition substantially enriched in elements with Z>14 as compared to the first
event (on December 6). While the December 6 event is similar in Fe/O to other events with
comparable fluence in solar cycle 23, the December 13 event has the highest Fe/O ratio of all
events with Si fluence > 100 (cm^ sr MeV/n)"'. In composition, this second event is most similar
to the event of November 6, 1997.
Keywords: solar energetic particles, composition, particle acceleration
PACS: 95.50.Vg, 96.50.Pw, 96.60.Q-
EVTRODUCTION
Since ACE was launched in 1997, the high-resolution particle spectrometers,
ULEIS [1] and SIS [2], have provided information on heavy ions from -0.1 to -100
MeV/nucleon for nearly 100 large solar energetic particle (SEP) events. The fluences
of these events span over 4 orders of magnitude (as measured > 12 MeV/nucleon) and
the Fe/O ratios vary by >2 orders of magnitude [3]. With the launch of the STEREO
spacecraft in 2006, it is now possible to make multi-point measurements of SEP
events and examine such spectra and composition variations as a function of position
within a single SEP event.
The STEREO spacecraft were launched in time to observe the last large SEP events
of solar cycle 23. Although the Sun had been very quiet throughout November 2006,
substantial activity returned when active region 10930 rotated over the east limb on
CP1039, Particle Acceleration and Transport in the Heliosphere and Beyond— 7^^ Annual Astrophysics Conference
edited by G. Li, Q. Hu, O. Verkhoglyadova, G. P. ZanJi, R. P. Lin, and J. Lulimann
O 2008 American Institute of Pliysics 978-0-7354-0566-0/08/$23.00
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December 5, 2006. As the region crossed the solar disk it produced 4 X-class and 5
M-class fiares, as well as at least 3 halo coronal mass ejections (CMEs). This solar
activity resulted in several SEP events; the two largest were associated with an X6.5
fiare on December 6 and an XI.5 fiare on December 13, which are the focus of this
study.
OBSERVATIONS
Both SEP events were well measured by the ULEIS and SIS instruments on ACE
and the LET sensor [4] of the IMPACT suite on the STEREO-B spacecraft. Figure 1
shows hourly oxygen intensities from the 3 instruments as a function of time for
energies ranging from 0.1 to 50 MeV/nucleon. The first SEP event occurred when the
active region was at ~E70 solar longitude and shows a classic eastern-event time
profile with a gradual intensity rise peaking near the shock at almost all energies [5].
The second event occurred when the region was at ~W25. In this event the intensities
at higher energies (e.g., >10 MeV/nucleon) exhibited fast rise times and peaked well
ahead of the shock passage while at low energies (<1 MeV/nucleon) they peaked
sharply at the shock.
•0.10MeV/n
•0.27MeV/n
•0.77MeV/n
-1.5 MeV/n
-4.2MeV/n
- 8.9 MeV/n
•18MeV/n
• 34 MeV/n
• 50 MeV/n
Shock
c
1 ULEIS
LET
c
0)
o
>
SIS
X
O
Dec 6
Dec 8
DedO Dec12 Dec14 Dec16
Date of 2006
FIGURE 1. Hourly oxygen intensities as a function of time from the ACE/ULEIS, STEREO-B/LET,
and ACE/SIS instruments. Interplanetary shock passages are marked by vertical lines and the times of
X-class flares are indicated at the top. The shaded regions indicate averaging intervals for abundances.
The two shaded regions in Figure 1 indicate the integration times used to calculate
the event-averaged spectra. It is possible that SEPs generated by the X-class fiare on
December 5 are included in the integration period for the first event, however as
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Figure 1 shows this contribution is negligible. Combined ULEIS + LET + SIS oxygen
and iron spectra for both events are shown in Figure 2 (left panel). It is immediately
apparent that the shapes of the spectra are distinctly different for the two events. The
eastern event on December 6 produced spectra that are gradually steepening with
increasing energy and that are nearly identical in shape for O and Fe. In contrast, the
western event of December 13 has hard portions of the spectra at high and low
energies with an inflection point near 5 MeV/nucleon. This odd shape is related to the
fact that the low energies peaked near the shock passage while the higher energies
peaked well before then.
10°
1
!
10"
10"
10°
r
r
0.01
•
A. 0 12/6
T Fe12/6
0.1
•
•
O 12/13
Fe 12/13
10
1
'
"
100
^
20
1000
Energy (MeV/n)
25
30
Z
FIGURE 2. Event-averaged oxygen and iron spectra for the two SEP events associated with flares on
12/6 and 12/13 (left). Abundances integrated from 12-60 MeV/n normalized to the SEP average
composition of Reames [7] as a function of nuclear charge for the two SEP events (right).
The two events also differ dramatically in their Fe/0 ratios, particularly as a function
of energy. This can be seen from the relative spacing of the O and Fe spectra over the
energy interval examined. While the December 6 event exhibits a roughly constant
Fe/0 ratio at all energies, the Fe/0 ratio increases dramatically with energy in the
December 13 event with ratios approaching unity above 10 MeV/nucleon (see also
[6]). Abundances of the elements from C to Ni were obtained for both events by
integrating the spectra from 12 to 60 MeV/nucleon. These values have been
normalized to the average values for large SEP events from Reames [7] and plotted
versus nuclear charge in the right panel of Figure 2. Here it is clear that the western
event is significantly enhanced in elements with Z>14 while the eastern event has
abundances that are similar to or slightly below the Reames average values.
These events can be put into the context of other large SEP events of solar cycle 23;
Figure 3 shows the event-integrated Si fluence (> 12 MeV/nucleon) as a function of
the event-averaged Fe/0 ratio (12-60 MeV/nucleon) for 98 events. The two December
2006 events are indicated by the red and blue symbols. The December 6 event is
fairly typical in its Fe/0 abundance for the events with largest fluences. In contrast,
the December 13 event has the highest Fe/0 ratio of any event with Si fluence above
100(cm^srMeV/n)"\
120
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10^
:
;
..
l-TTTTl
10^ r
% •
•
•
•
••
•
.
:
•
•
•
•j
•
•
•
• • • •
••
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10^ r
•
•-•H
w
•
1
• •
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IC
;
•
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•
10^ r
l-t
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. • • <•
•• • • - ;
•
10"
0.001
-
•
•
•
•
•;
•
-1
1 LJ
0.1
0.01
1
10
Fe/0 (12-60 MeV/n)
FIGURE 3. Silicon fluences (for 12-60 MeV/n) versus Fe/0 abundances (for 12-60 MeV/n) for 98
large SEP events from September 1997-December 2006. The December 6 event is plotted as a blue
square and the December 13 event as a red square. The green diamonds indicate events identified by
Leske et al. [8] as large impulsive events.
DISCUSSION
The SEP event on December 13 is unusually rich in elements with Z>14. In its
elemental composition it is most similar to one of the first events measured by ACE
instruments, November 6, 1997 [9,10]. Although the composition of the November 6
event was unexpected at the time, several additional substantially Fe-enriched events
were observed as solar maximum approached [9]. Since then much work (theoretical
and observational) has been done to understand the origin of the enrichment, leading
to a general consensus that fiare-accelerated particles are involved. Still being debated
is how these particles are incorporated into the observed SEP events.
The scenario put forth by Tylka et al. [11] includes fiare particles through the
suprathermal seed population available to the CME-driven shock that accelerates
particles to SEP energies [10]. Whether a given SEP event is Fe-rich or not is a
function of a) the presence of fiare suprathermals (in addition to the solar wind
suprathermals that are continually present) and b) the orientation of the shock.
Criterion (b) affects the accelerated population because quasi-perpendicular shocks are
expected to have a higher injection threshold energy and so would inject fewer solar
wind suprathermals (which dominate at lower energies) as compared to the fiare
suprathermals (which would dominate at higher energies). Thus a quasi-parallel shock
is expected to produce an SEP event with composition typical of most large SEP
events (e.g., [7]), regardless of the presence of fiare suprathermals. In contrast, a
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quasi-perpendicular shock would produce an Fe-rich SEP event if there were
significant flare suprathermals present in the seed population available to the shock.
The scenario of Cane et al. [12,13] involves a competition between two SEP
sources: flare-accelerated and shock-accelerated. The composition of the shockaccelerated SEPs would be similar to that of average SEP events (or lower at higher
energies due to diffusion effects) while the flare-accelerated SEPs would have
composition enriched in elements with Z>10. Which component dominates in an
observed SEP event depends on the relative strength/size of each component as well as
the magnetic connection of the observer to the flare site. The flare-accelerated SEP
component is most readily apparent for observers that are well connected to the flare
site and measuring particles at energies above which the shock component contributes
heavily, typically ~25 MeV/nucleon.
A final option adds a step beyond the Cane et al. idea. Li and Zank [14] suggest
that the flare-accelerated particles of Cane et al. can also scatter in the interplanetary
medium and be reflected back towards the Sun where they can encounter the
interplanetary shock and be reaccelerated. Thus the composition observed by an
observer would depend strongly on their magnetic connection to the shock and the
flare site as well as the amount of interplanetary scattering of the flare particles.
The fact that the pair of December 2006 SEP events originated from the same
active region but were separated enough in time (and longitude) to individually be
relatively isolated appears to present an opportunity to examine the relative merits of
the scenarios just described. Unfortunately, without near-Sun measurements of the
seed population and shock characteristics it is difficult to make definitive statements
regarding the shock-orientation scenario of Tylka et al. What can be surmised,
assuming that the scenario is correct, is that the event on December 6 was either
generated by a quasi-parallel shock or that there was an insufficient amount of flare
suprathermals, while the December 13 event was created by a quasi-perpendicular
shock acting on a seed population that included a significant amount of suprathermal
flare particles. It is known that there were 3 C-class flares (no M- or X-class) in the 3
days prior to the event on the 13th that might have been a source for flare
suprathermals. For the December 6 event there were many more flares in the previous
3 days (21 C-class, 1 M-class, and 1 X-class) making a flare suprathermal population
more likely; suggesting that most SEPs observed at Earth were accelerated by a quasiparallel shock for this event. Although the shock orientations near the Sun are
unknown, ACE observations indicate both shocks were quasi-parallel at 1 AU
(12 ± 10° on December 8 and 36 ± 19° on December 14).
The compositional differences between the two events are consistent with the
scenarios of Cane et al. and Li and Zank in that the Fe-rich event was nominally well
connected magnetically with the observing spacecraft while the Fe-average one was
not. Additionally the increase in the Fe/0 ratio with increasing energy in the western
event of December 13 is consistent with a directly observed flare component
dominating at higher energies as suggested by Cane et al. One possible caveat
concerns the strong shock that accompanied this event. Cane et al. point out that an
exceptionally fast/strong shock can overwhelm the flare component at higher energies
even when an event is well connected to the observer. The transit speed of the shock
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associated with the SEP event on December 13 was -1200 km/s putting it slightly
over the -1000 km/s discriminator level used by Cane et al. [13].
Another measure that is commonly used to identify flare material is the ^He/'He
ratio. Using 0.5-2 MeV/nucleon data from the ULEIS instrument the ^He/''He ratio for
the first event was determined to be 0.0011 ± 0.0005 while analysis of the second
event resulted in an upper limit of 0.001. At higher energies, SIS data from 8-15
MeV/n provide only upper limits of 0.01 for both events. These results are rather
surprising as many events with enhanced Fe/0 abundances also have enrichments in
^He/'He over the solar wind value of 5 x lO""* [9,10,15], thus one would expect the Ferich December 13 event to have an enhanced ^He/''He ratio. However, the November
6, 1997 event, which is most compositionally similar to the December 13 event, had a
^He/'He ratio of 0.0021 ± 0.0008, only a factor of 4 above the solar wind value [10].
The STEREO mission is designed to provide multi-point observations of SEP
events and will provide measurements critical to testing longitudinal-dependent
scenarios such as those of Cane et al. and Li and Zank. Unfortunately, although many
of the STEREO instruments were operational and observed these December events,
the two spacecraft were still in Earth orbit and not sufficiently separated in longitude
from each other or from ACE to provide the desired multiple views of the events.
Thus one must continue to wait until other large, Fe-rich events, similar to December
13, are simultaneously observed from the different vantage points of STEREO and
ACE to properly examine these proposed scenarios.
ACKNOWLEDGMENTS
This work was supported by NASA under grants
NNX06AC21G, and by NSF under grant ATM-0454428.
NAG5-12929
and
REFERENCES
1. G. M. Mason, et al., Space Sci. Rev., 86, 409-448 (1998).
2. E. C. Stone, et al.. Space Sci. Rev., 86, 357-408 (1998).
3. R. A. Mewaldt, Space Sci. Rev. 124, 303-316 (2006).
4. R. A. Mewaldt, st al. Space Sci. Rev. (2007), doi:10.1007/sll214-007-9288-x.
5. H. V. Cane, et al., J. Geophys. Res. 93, 9555-956 (1988).
6. C. M. S. Cohen, et al., Proced. Internat. Cosmic Ray Conf. (Merida), (2008), in press.
7. D. V. Reames, Space Sci. Rev. 90, 413-491 (1999).
8. R. A. Leske, et al., Proced. Internat. Cosmic Ray Conf. (Tskuba), 3253-3256 (2003).
9. C. M. S. Cohen, et al., Geophys. Res. Lett 26, 2697-2700 (1999).
10.G. M. Mason, etal., Geophys. Res. Lett. 26, 141-144 (1999).
11. A. J. Tylka, sXa\.,Astrophys. J. (2005), doi:10.1086/429384.
12.H.V. Cane, et al., Geophys. Res. Lett (2003), doi:10.1029/2002GL016580.
13.H.V. Cane, etal., J. Geophys. Res. (2006), doi:10.1029/2005JA011071.
14.G.Li and G. P. Zank, Geophys. Res. Lett (2005), doi:10.1029/2004GL021250.
15.M. I. Desai, sXa\.,Astrophys. J. (2006), doi:10.1086/505649
123
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